Mention has been made in the appropriate sections of coating the backs of stones to heighten color, improve phenomenal effects, or conceal defects. The coat1hg of off-color diamonds to improve color is discussed in the diamond course. There remains to discuss the coating of colored stones by other means, in order to improve or produce color.
One such method that has been used to heighten the color of blue zircons consists of applying to the pavilion surface by a high-vacuum process a transparent, wear-resistant coating of a fluoride compound. Heating in acid softens the coating sufficiently so that it can be scrapped off to reveal the actual pale color underneath.
On several occasions, Geologists has encountered stones that appeared to be emeralds, but that were proved by examination to be nearly worthless flawed aquamarine with a green plastic coating. A pair of large 'emerald' drops was purchased by a manufacturing jewelry for more than one thousand dollars and an elaborate pair of earrings was made for them. Only when the lapidary enlarged the holes of the stones and discovered that his drill removed a plastic material was the nature of the fraud suspected. The buyer of the stones had "protected" himself by looking at them through an emerald filter. Later, after the plastic coating had been removed in the laboratory, it has found that the green plastic itself turned red under the filter and that the stones themselves were merely poor-quality beryl. Carved "emeralds" and a string of "emerald" beads of precisely the same type have been encountered, as well.
A European firm has developed a method of growing a coating of synthetic emerald on large faceted stones of colorless or pale Brazilian beryl. The coating, which is approximately one millimeter thick, is formed in approximately eight days. It is clear that a hydrothermal process is used. After the stones receive the artificial layer, they are then re-polished. In order to produce an attractive finish and yet leave enough of the thin coating to resemble pale but attractive emerald, the re-polishing operation must be carried out carefully. No commercial name has yet been given to the product.